Effect of carbon surface chemistry on the adsorption of aromatic compounds from dilute aqueous solutions

Abstract

In order to tailor design activated carbons for specific applications, this work is undertaken to gain an improved fundamental understanding on the adsorption mechanism of aromatic compounds, in particular the effects of the surface chemistry of activated carbons, solute properties and solution pH. This is carried out with the aim of improving the current understanding of their adsorption behaviour in aqueous solutions. To achieve the objectives, the adsorption of four dissociating and one non-dissociating aromatic compounds onto activated carbons from dilute aqueous solutions are investigated. To determine the orientation of the adsorbate molecules on the activated carbons surfaces, a combination of theoretical and experimental studies was used. The theoretical approaches used in this work involve using the Cerius2 package to determine the area of each molecule in the XY (facedown), XZ, and the YZ planes. The experimental approaches involve obtaining adsorption isotherms of the solutes at various pHs. The isotherms are then fitted to the homogeneous Langmuir equation to obtain the maximum monolayer capacities for all solutes. Using the calculated areas together with the monolayer adsorption capacities obtained at pH 2, it is found that the orientation of the sorbates on the activated carbon surface is edge-to-face, tilted at various angles, and not facedown, as previously thought. This shows that the aromatic compounds could not interact with the activated carbons through n-n interactions. This finding was further confirmed after performing a series of ab initio calculations using Gaussian98. This reveals that determination of the maximum adsorption capacities of activated carbons for aromatic compounds is not a simple task. Furthermore, its determination from the BET surface area of the activated carbon and the average molecular area would be erroneous. The effects of the surface heterogeneity are modelled using the theory of localized physical adsorption, with the Langmuir equation as the local isotherm equation, and assuming a Gaussian adsorption energy distribution. Comparison of the homogenous and heterogeneous model fits, together with the pattern in the spread of the adsorption energies reveals that under alkaline conditions, it is mainly the high energy sites that take part in the adsorption process, as low energy sites are no longer available for adsorption. Furthermore, it is also found that the main parameters, monolayer adsorption capacity, affinity coefficient and the heterogeneity of the activated carbons depend on the ratio of the anionic to molecular species in solution, irrespective of the solute. These relationships are used to model the Langmuir parameters at other pHs. ………………….